Videokeratoscopes heretofore obtained relative curvature readings from the cornea. They recorded specular reflections from the partially reflecting corneal surface and sought to model the surface by treating it as a convex mirror.
Typically videokeratoscopes illuminate the eye with a set of concentric rings and attempt to recreate the surface from the relative sizes of the rings as reflected from the cornea. The ring sizes have been used to document the relative surface curvature. However, attempts to reconstruct the three dimensional coordinates of the surface have had to make a priori assumptions about the cornea's shape. Since the shape of any particular person's cornea does not necessarily match the a priori assumed shape upon which the reconstruction calculation is based, the resulting topographic map can be erroneous. Also, there is an inability to measure the peripheral cornea, since ring mires projected and imaged from in front of the eye are obscured by facial features, which limit the extent of the image obtained from the periphery of the cornea. There is reduced accuracy in the central cornea because of both the absence of a central data point and because the absolute size of the ring mires within the central 3 mm are small. Sufficiently irregular corneas cannot be measured, because irregular corneal surfaces produce distorted images that cause neighboring rings to merge with each other, thus preempting image quantification.
Machine vision systems have been used to map a uniform and diffusely reflecting surface of an object. Structure was provided within an image of the surface by projecting a calibrated structured light pattern onto a front surface and obtaining an image of the projected pattern from a direction different than that of the projector. Computer algorithms are used to extract features from the projected light pattern and calculate the elevation of the surface from the elevation dependent displacements of the extracted features. However, structured light machine vision systems designed to map opaque objects may be inaccurate when used on translucent objects, due to alterations within the imaged pattern caused by light being imaged that arises from beneath the front surface.
Structured light topographic mapping systems have employed a fluorescent substance to map the corneal surface. These systems have assumed that the layer of fluorescing material, i.e. the tear film, is thin and that the substrate of the cornea, i.e. the corneal stroma, is not fluorescing. However, these assumptions do not necessarily hold in practice. The tear film varies in thickness, e.g. the tears pool at the lid margins and fill small irregularities in the corneal surface. In addition, the corneal stroma absorbs sodium fluorescein whenever the corneal epithelium is compromised such as following injury, in disease states, and during eye surgery.
Structured light topographic mapping systems employing a fluorescent substance have not addressed the alterations in an image caused by thick fluorescing tear films or a fluorescing corneal stroma.
Thus, there remains a need for an apparatus and method that can accurately measure the three dimensional coordinates of the corneal surface in the presence of tear film pooling and/or stromal fluorescence. In addition, there remains a need for an apparatus and method that can measure the thickness of the cornea over the entire extent of the cornea, and match the thickness profile of the entire cornea with the surface topography of the cornea.